I don’t think this is a PLP issue. I think it’s a local WP or theme settings issue.
Since the demo is as “WP default” as possible, I suspect this might be a theme issue. That is, your theme must be customizing the way paginated URL’s read. I’m not entirely sure why a theme author would do this, but s/he must have had reasons.
Here’s a post from stack exchange that outlines one method for customizing pagination URL’s: